bug#48425: Should #nil be equal? to '()?

[Taylan Kammer]

I believe it would be better if #nil were equal? to ().

It would keep *not* being equal? to #f and as such not disturb the
property of transitiveness.

Making #nil and () be equal? would be a lot more intuitive since
they both represent the empty list, and since equal? is commonly
used to test the equality of lists.  Meeting this expectation would
probably prevent a common type of unexpected behavior where a list
coming from Elisp code is not equal? to a list coming from Scheme
code, even though they have the same contents.

Attached is a patch to realize the change.  Note that it increases
the size of compiled code that uses equal?.  I don't know if this
represents a problem or not.

Before patch:

scheme@(guile-user)> ,disassemble (lambda (x y) (equal? x y))
Disassembly of #<procedure 55dd585a0c58 at <unknown port>:1:13 (x y)> at 
#x55dd585a0ad4:

   0    (instrument-entry 131)                                at (unknown 
file):1:13
   2    (assert-nargs-ee/locals 3 0)    ;; 3 slots (2 args)
   3    (eq? 1 0)                                             at (unknown 
file):1:27
   4    (je 29)                         ;; -> L4
   5    (immediate-tag=? 1 7 0)         ;; heap-object?
   7    (jne 22)                        ;; -> L3
   8    (immediate-tag=? 0 7 0)         ;; heap-object?
  10    (jne 15)                        ;; -> L2
  11    (static-ref 2 96)               ;; #f
  13    (immediate-tag=? 2 7 0)         ;; heap-object?
  15    (je 7)                          ;; -> L1
  16    (call-scm<-scmn-scmn 2 103 107 113)
  20    (static-set! 2 87)              ;; #f
L1:
  22    (scm-ref/immediate 2 2 1)       
  23    (handle-interrupts)             
  24    (tail-call)                     
L2:
  25    (make-immediate 2 4)            ;; #f
  26    (reset-frame 1)                 ;; 1 slot
  27    (handle-interrupts)             
  28    (return-values)                 
L3:
  29    (make-immediate 2 4)            ;; #f
  30    (reset-frame 1)                 ;; 1 slot
  31    (handle-interrupts)             
  32    (return-values)                 
L4:
  33    (make-immediate 2 1028)         ;; #t
  34    (reset-frame 1)                 ;; 1 slot
  35    (handle-interrupts)             
  36    (return-values)                 

After patch:

scheme@(guile-user)> ,disassemble (lambda (x y) (equal? x y))
Disassembly of #<procedure 55b741d3ad50 at <unknown port>:8:13 (x y)> at 
#x55b741d3ab94:

   0    (instrument-entry 145)                                at (unknown 
file):8:13
   2    (assert-nargs-ee/locals 3 0)    ;; 3 slots (2 args)
   3    (eq? 1 0)                                             at (unknown 
file):8:27
   4    (je 43)                         ;; -> L6
   5    (immediate-tag=? 1 3583 260)    ;; null?
   7    (jne 12)                        ;; -> L2
   8    (immediate-tag=? 0 3583 260)    ;; null?
  10    (je 5)                          ;; -> L1
  11    (make-immediate 2 4)            ;; #f
  12    (reset-frame 1)                 ;; 1 slot
  13    (handle-interrupts)             
  14    (return-values)                 
L1:
  15    (make-immediate 2 1028)         ;; #t
  16    (reset-frame 1)                 ;; 1 slot
  17    (handle-interrupts)             
  18    (return-values)                 
L2:
  19    (immediate-tag=? 1 7 0)         ;; heap-object?
  21    (jne 22)                        ;; -> L5
  22    (immediate-tag=? 0 7 0)         ;; heap-object?
  24    (jne 15)                        ;; -> L4
  25    (static-ref 2 96)               ;; #f
  27    (immediate-tag=? 2 7 0)         ;; heap-object?
  29    (je 7)                          ;; -> L3
  30    (call-scm<-scmn-scmn 2 103 107 113)
  34    (static-set! 2 87)              ;; #f
L3:
  36    (scm-ref/immediate 2 2 1)       
  37    (handle-interrupts)             
  38    (tail-call)                     
L4:
  39    (make-immediate 2 4)            ;; #f
  40    (reset-frame 1)                 ;; 1 slot
  41    (handle-interrupts)             
  42    (return-values)                 
L5:
  43    (make-immediate 2 4)            ;; #f
  44    (reset-frame 1)                 ;; 1 slot
  45    (handle-interrupts)             
  46    (return-values)                 
L6:
  47    (make-immediate 2 1028)         ;; #t
  48    (reset-frame 1)                 ;; 1 slot
  49    (handle-interrupts)             
  50    (return-values)                 


- Taylan

0001-Make-nil-and-equal-as-per-equal.patch
Description: Text document